2000 Chevrolet Prizm Questions

About 125,000 miles on it. Owned it since 90k roughly. Every once in a while the engine light would blink on, seemingly random (solid 55 straightaways, usually), then on next day startup or so, would be off. This time it turned on about a week after my last oil change (due for another soon), and has been on since. Had the code checked and it's the same one it's always thrown when I go to have it checked. First guy (bout a year ago) told me system too lean- took out the mass air flow sensor and cleaned it out, was just fine after. Now, someone told me Cat converter is clogged- this is just off of reading his code reader and seeing PO420. Dunno how accurate any of that is. I'm wanting to know the reason and what I should do in steps to figure it out before I go dumping my money into incorrect solutions.-I don't know a whole lot about cars, or a good mechanic around here as I recently moved.